EVOLUTION OF LYMAN-LIMIT ABSORPTION SYSTEMS OVER THE REDSHIFT RANGE 0.40-LESS-THAN-Z-LESS-THAN-4.69

We present the results of a study of 15 z > 4.2 QSOs that extend statistical studies of Lyman-limit absorption line systems [N(H I) greater-than-or-equal-to 1.6 x 10(17) cm-1] to the highest redshifts currently possible. This data set has been combined with homogeneous data sets of low-redshift Hubble Space Telescope observations and intermediate-redshift ground-based observations. Assuming a power law of the form N(z) = N0(1 + z)gamma for the number density, we find gamma = 1.55 and N0 = 0.27, with N = 3.27 per unit redshift at z = 4. The >99.7% confidence limits for gamma are 2.37 and 0.82. For the first time this indicates intrinsic evolution of these absorbers for an OMEGA = 1 universe (gamma = 1/2 for no evolution). This result is marginally consistent with no evolution for OMEGA = 0 (gamma = 1 for no evolution). These results differ significantly from those of Sargent, Steidel, & Boksenberg (1989) who found no intrinsic evolution in Lyman-limit systems up to z = 3.5, and Lanzetta (1991) who found much stronger evolution for z > 2.5. stronger evolution for z > 2.5. For z less than or similar to 2 the space density of Lyman limit systems [N(1.5) almost-equal-to 1.1] and Mg II absorbers with rest equivalent width W0 > 0.3 angstrom [N(1.5) almost-equal-to 1.0] is almost identical. This supports the picture that Lyman-limit and Mg II absorbers are drawn from the same population.